Purely hydraulically-actuated steering systems, particularly for vehicles with four steered wheels, are known which substantially comprise two steered front wheels and two steered rear wheels, which are associated with a respective actuation mechanism operated by a fluid medium (oil), a tank for the fluid, and a pump that feeds the fluid drawn from the tank to a hydraulic power steering unit, which controls its flow along a line for transmission to the two actuation mechanisms, a line for returning the fluid to the tank by means of said hydraulic power steering unit being provided.
The hydraulic power steering unit is controlled externally by means of a steering column provided with a control wheel.
Each one of the two actuation mechanisms is constituted by a double-acting hydraulic cylinder, the stem of which reaches outside both opposite ends of the cylinder body in order to actuate a respective wheel, the chambers of the two cylinders being provided with a respective opening that is connected selectively to the chambers of the other cylinder, to the delivery from the hydraulic power steering unit, and to the return to said unit.
Devices for reversing the flow of the fluid and/or valves and/or distribution units that are suitable to select the steering modes, with two or four wheels, can optionally be provided along the transmission line.
In the mode with four steered wheels, the fluid, sent under pressure by the pump to the hydraulic power steering unit, is sent to feed a chamber of one of the two hydraulic cylinders, the fluid in output from the opposite chamber of that cylinder is sent, by means of the transmission line, to feed a chamber of the other hydraulic cylinder, while the fluid in output from the opposite chamber of this last cylinder is sent to the return line, which leads into the tank by means of the hydraulic power steering unit.
Depending on the steering direction, one of the two hydraulic cylinders acts as an actuator for the other cylinder.
These known types of purely hydraulically-actuated steering systems are not free from drawbacks, including the fact that they are affected by bleeding of fluid, which causes misalignments between the front wheels and the rear wheels and therefore cause incorrect steering.
Bleeding occurs in particular within the two hydraulic cylinders and are generally different between the two cylinders, and is due for example to wear of the corresponding sliding components, to dimensional tolerances, et cetera.
In order to obviate this drawback, it is known for example to lock the rear wheels in a “straight” configuration, i.e., parallel to the longitudinal axis of the vehicle, and to “manually” align the front wheels with them by acting on the steering system and checking the position of the front wheels with respect to the rear wheels by means of a simple visual check on the part of operators.
This “manual” method is disadvantageously labor-intensive, time-consuming and inaccurate, since it is entrusted to the skill of the operators; moreover, it is not feasible if the wheels of the vehicles are even only partially covered by a protective housing, such as for example the wheels of certain vehicles for agricultural use or for earth-moving.
Electronic control systems are also known which are substantially constituted by means (sensors) for detecting the steering angle of the front wheels and by means (sensors) for detecting the steering angle of the rear wheels, which are functionally associated with a processing unit, which computes any discrepancy between the two detected values, emitting a corresponding output signal.
The output signal can control, for example, a warning device that reports the detected discrepancy to the operator and therefore reports the need to perform a “manual” realignment of the wheels.
As an alternative, the output signal can control a correction device, which is inserted along the distribution line and by means of which the flow-rate of fluid to be sent to the two hydraulic cylinders in order to realign the wheels can be modified periodically or depending on a preset threshold value of the computed discrepancy.
These electronic control systems are not free from drawbacks, including the fact that they are very complex and expensive, the fact that they do not allow “constant” or “continuous” wheel realignment but only a periodic or occasional realignment, since it is entrusted either to the voluntary intervention of the operator or to the activation of the control device, and the fact that if they merely report to the operator the need to perform realignment, said realignment depends on the skill and sensitivity of said operator.
Finally, automatic realignment devices are known which consist in providing, on the internal wall of the body of the hydraulic cylinders and proximate to their opposite ends, a groove that cooperates with the sealing gasket of the respective piston.
When the piston has reached the end of its stroke, which is generally set by an external locking of the steering lever that connects one end of the stem of the hydraulic cylinder to the respective wheel, the sealing gasket of the piston is arranged at the groove.
Accordingly, a certain amount of oil bleeds from the active chamber of the hydraulic cylinder toward the groove thus exposed, in order to be fed, via a duct for connection to the distribution line, into the active chamber of the other hydraulic cylinder and thus take up any misalignments.
However, even these known automatic realignment devices are not free from drawbacks, including the fact that their operation is inaccurate and inconstant, as it in fact depends on many variables, including for example the dimensional tolerances of the hydraulic cylinders, the elasticity of the gasket used, and the physical characteristics of the fluid used (viscosity, temperature, et cetera), which can vary in different operating conditions.
Another drawback of known automatic devices is that even if the grooves are finished and blended accurately to the internal wall of the cylinder body, they cause abnormal and frequent wear and damage of the piston gasket, which accordingly has a shorter average life and therefore must be replaced frequently.